General Phenylpropanoid Metabolism

PAL catalyzes the deamination of L-phenylalanine and produces f-cinnamic acid in the first step in phenolic metabolism, so PAL is referred to as the key enzyme or rate-limiting enzyme in phenolic biosynthesis. It is difficult to compare PAL activity in different circumstances, because of the large effects of the physiological stage on the enzyme and because of its sensitivity to external factors such as light, temperature and stress (Macheix et al. 1990). 

---

The shikimate/arogenate pathway leads to the formation of three aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophane. This amino acids are precursors of certain homones (auxins) and of several secondary compounds, including phenolics [6,7]. One shikimate/arogenate is thought to be located in chloroplasts in which the aromatic amino acids are produced mainly for protein biosynthesis, whereas the second is probably membrane associated in the cytosol, in which L-phenylalanine is also produced for the formation of the phenylpropanoids [7]. Once L-phenylalanine has been synthesized, the pathway called phenylalanine/hydroxycinnamate begins, this being defined as "general phenylpropanoid metabolism" [7]. 

Fig. 11.1 Simplified diagram of the flavonoid biosynthetic pathway, starting with the general phenylpropanoid metabolism and leading to the main types of flavonoids. Only a few examples are illustrated of the large variety of flavonoids that arise through modification at different positions (not indicated or shown as R). Enzymes catalysing some key reactions are indicated by the following abbreviations PAL, phenylalanine ammonia-lyase CHS, chalcone synthase CHI, chalcone isomerase DFR, dihydroflavonol reductase F3H, flavanone 3-hydroxylase F3 5 H, flavonoid 3 5 -...

Fig. 1. Presumed biosynthetic pathway of glyceollin and its structural isomers in soybean. The sequence comprises the enzymes of general phenylpropanoid metabolism ...

The aromatic compound raspberry ketone, 4-(/ -hydroxyphenyl)-2-butanone, also known disframbinone, has been identified as the main fiavor impact component of raspberries, where it is contained in very low concentrations (0.1-0.2 ppm), only during the late maturation stage of the berry [140]. The biosynthetic pathway (shown in Scheme 9.6) stems from the general phenylpropanoid metabolism, starting from/)-coumaroyl-Co A and malonyl-CoA, involving at least two different enzymes (a synthase and a reductase) [141]. 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

The general phenylpropanoid pathway begins with the deamination of L-phenylalanine to cinnamic acid catalyzed by phenylalanine ammonia lyase (PAL), Fig. (1), the branch-point enzyme between primary (shikimate pathway) and secondary (phenylpropanoid) metabolism [5-7]. Due to the position of PAL at the entry point of phenylpropanoid metabolism, this enzyme has the potential to play a regulatory role in phenolic-compound production. The importance of this is illustrated by the high degree of regulation both during development as well as in response to environmental stimuli. 

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

One of the major features of phenylpropanoid metabolism is the diversity of end products. The set of enzymic reactions leading from phenylalanine to 4-coumaroyl coenzyme A is common to pathways which lead to these diverse end products and is known as the general phenylpropanoid pathway (Fig. 1). Those biochemical reactions which lead to the synthesis of specialised products are known as branch pathways. 

It is reasonable to assume from the available evidence that the enzyme acts at a switching point in metabolism and diverts L-phenylalanine from the general pool of amino acids used for protein synthesis to the biosynthesis of phenylpropanoid compounds. Since initial steps are probable sites for overall pathway regulation, it is therefore not surprising that the factors which influence L-phenylalanine ammonia lyase activity have been subject to detailed scrutiny. Thus phytochrome control in dark grown seedlings. 

---